At two hundred that is incorrect as it is compounding. You get the 30 coins plus everything else you have already gotten, which is 10+15+20+25+30, as it gets the thirty coins. By doing 200 you get a total of 100 coins from rewards. Where if you do a fifty you get 25, at which point the ratio is sound, but at 250 it becomes more efficient as the higher your combo rate gets the more money you get per zombie killed.
After unlocking the secret level I straight up giggled with joy at the start of it. Hearing what they said at the start was amazing after killing all of the others one by one....
 "Coaster Racer 3 Beta"){kind=icon}
 "Pretentious Game 2"){kind=icon}
 "GrindQuest Censored"){kind=icon}
 "Caribbean Admiral"){kind=icon}
 "CaribbeanAdmiral"){kind=icon}
 "This is not a minimalist game"){kind=icon}
 "Ninja Slash"){kind=icon}
 "Pandemic 2"){kind=icon}
 "Caravaneer"){kind=icon}
 "no-one has to die."){kind=icon}
 "Bear Grills"){kind=icon}
 "Jim Loves Mary"){kind=icon}
 "MadBurger"){kind=icon}
 "Transformice"){kind=icon}
 "Fear Less!"){kind=icon}
 "Card Monsters"){kind=icon}
 "Avant-Garde"){kind=icon}